This invention relates to a method of detecting abnormality in an exhaust gas concentration sensor for use in an internal combustion engine equipped with a fuel supply control system, which controls the air-fuel ratio of a mixture to be supplied to the engine in a feedback manner responsive to an output from the sensor.
It has conventionally been carried out to detect the concentration of a specific component, e.g. oxygen contained in exhaust gases emitted from an internal combustion engine, and set the value of an air-fuel ratio correction coefficient in response to the detected concentration value, and correct a basic fuel supply quantity by the set correction coefficient to thereby control the air-fuel ratio of an air-fuel mixture being supplied to the engine so that it is maintained within a certain range with a desired air-fuel ratio value as the median value. As such a sensor for detecting the oxygen concentration an oxygen concentration sensor (hereinafter called "O.sub.2 sensor") has generally been employed, which is formed of a solid electrolyte of zirconia (ZrO.sub.2) for example. This typ O.sub.2 sensor has such a characteristic that its electromotive force suddenly changes as the air-fuel ratio of a mixture supplied to the engine varies across a stoichiometric mixture ratio in such a manner that the sensor output voltage assumes a higher level and a lower level than a predetermined reference output voltage, respectively, when the air-fuel ratio is richer than the stoichiometric mixture ratio and when the former is leaner than the latter. Electric disconnection in the O.sub.2 sensor of this type or in the wiring thereof or deterioration of the sensor exerts a great influence upon the accuracy of the air-fuel ratio control. Therefore, it is necessary to always monitor the operation of an exhaust gas component concentration detection system including the O.sub.2 sensor so as to ensure normal functioning of the air-fuel ratio control system based upon a normal output from the sensor.
Various methods for detecting abnormality in such exhaust gas concentration sensors have heretofore been proposed. For example, a method has been proposed by Japanese Provisional Patent Publication (Kokai) No. 58-222939, which measures the interval of time from an instant the value of an air-fuel ratio correction coefficient changes stepwise to an instant it again changes stepwise, i.e. the time interval from the time the sensor output is inverted from a rich side to a lean side or vice versa with respect to a predetermined reference level, decides that the sensor is faulty if the measured time interval is longer than a predetermined period of time, and then sets the air-fuel ratio correction coefficient value to a predetermined value and corrects a basic fuel supply quantity by the set coefficient value.
Another abnormality detection method has been proposed by Japanese Provisional Patent Publication (Kokai) No. 59-3137, which determines whether the value of an air-fuel correction coefficient falls outside a normal range defined by upper and lower limit values that can be assumed by an exhaust gas concentration sensor during operation of an internal combustion engine functioning normally, and when the coefficient value falls outside the normal range, measures the time elapsed from the time the coefficient value shows a value outside the normal range for the first time, and decides that the sensor is malfunctioning if the measured elapsed time exceeds a predetermined time period.
These conventionally proposed methods have the disadvantage that when the engine is operating in a low load condition such as an idling condition, the temperature of an O.sub.2 sensor applied is so low that the sensor has not been activated as yet with its output level being uncertain or unstable, sometimes outputting a rich or lean value which does not correctly represent the actual air-fuel value, resulting in failure of accurate air-fuel ratio control, and if the abormality detection is carried out when the sensor output is still uncertain or unstable, it can result in a wrong diagnosis that the sensor is functioning abnormally even if it is actually operating normally. Therefore, these conventional methods cannot be carried out before the O.sub.2 sensor is completely activated, and as a result, the time period before abnormality of the sensor can be accurately detected after the start of the engine is considerably long, which can cause degraded exhaust emissions from the engine due to the failure of the sensor to properly function.
An abnormality detection method is also known from Japanese Provisional Patent Publication (Kokai) No. 53-95431, which decides that an exhaust gas concentration sensor such as an O.sub.2 sensor is faulty if the output voltage from the sensor assumes a very high value, e.g. 6 volts. According to this method, although a disconnection in the sensor or in the wiring can be correctly detected, a short-circuit in the sensor or in the wiring cannot be detected because the output voltage should then drop to 0 volt.